Biology Reference
In-Depth Information
(Manchester et al. 2006), adding to the inventory of angiosperms in fresh-
water marsh and riverine vegetation. MacGinitie 1969 calls the vegetation
Orizaban in analogy with the seasonally dry woodland-savanna on Mount
Orizaba in Mexico; Wolfe 1985 calls it a semideciduous tropical dry forest
similar to the short-lived middle Eocene seasonally dry tropical forest of
the southeastern United States (Bullock et al. 1995; Graham and Dilcher
1995). The Copper Basin and the Green River fl oras show the mosaic of
vegetation types of increasingly modern aspect at the formation level that
were developing in the American west during the middle to late Eocene.
Further details are contributed by other large and important fossil fl oras
in the region that are often associated with extensive faunas. The Floris-
sant fl ora to the southeast in central Colorado is dated to the late Eocene
or early Oligocene, around 35 Ma (Gregory-Wodzicki 2001; Leopold and
Clay-Poole 2001; Manchester 2001; Meyer 2001; Meyer and Smith 2008).
It includes a rich mesic forest growing along streams and lakes, and a drier
evergreen oak-pine woodland on the slopes. The MAP is estimated at 508-
635 mm and winter dry, and the MAT at 18°C (MacGinitie 1953) to a seem-
ingly low 12.5°C (CLAMP) depending on the paleoelevation assigned to the
site (I, 204-10). The Beaverhead Basins fl oras (Becker 1961, 1969) are to
the northwest in southeastern Montana and are also late Eocene to early
Oligocene in age. They show a similar aquatic, lake and riverside (
Typha
),
and fl oodplain vegetation (
Glyptostrobus, Taxodium, Salix
) in the lowlands,
a mesic deciduous forest on the slopes (
Ginkgo
,
Metasequoia
,
Sequoia
,
Acer
,
Quercus
;
Fraxinus
, ash;
Ulmus
;
Zelkova
, an Asian tree related to elm), and a
western montane coniferous forest at the highest elevations (
Abies
,
Picea
).
Also present were a substantial number of drier-habitat plants such as
Ju-
niperus
(juniper),
Arctostaphylos
,
Berberis
(barberry),
Cercocarpus
,
Mahonia
,
and
Potentilla
. The driest of the fl oras in the Beaverhead Basins is the Ruby
fl ora, which is also the youngest (and so further along the trend toward
dryness and seasonality), and it adds
Ephedra
(Mormon tea, an extreme
xerophyte) and
Prosopis
(mesquite). These assemblages show that by the
end of the Eocene, versions of a shrubland/chaparral-woodland-savanna
ecosystem and elements of a desert vegetation occupied the coarse soils,
rain shadow habitats, and steeper slopes of the drier intermontane basins of
the Rocky Mountain region, and they would coalesce and spread in the later
Tertiary and Quaternary.
To the far northeast, at about 81°N, fossil fl oras (Axelrod 1984) and fau-
nas (McKenna, 1983) provide an estimate of 13°C MAT in the middle to
late Eocene, while in the southeastern United States at about 35°N MAT
was 24°C, for a gradient of 0.28°C/1° latitude. The present gradient is
